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38 CLOTHING TEST METHODS same 5°C. difference of temperature would then be reduced to about half at the lower temperatures. The answer to this objection is that the heat loss by convection also changes with the ambient temperature but in the opposite sense, and the resultant total heat loss in ordinary cases, where emissivity of surfaces is high, is practically unchanged at low temperatures. Heat loss by convection, for a given velocity of air movement, increases as the density of the air increases with lowered temperature. Total Heat Loss, Insulation of the Air' In human calorimetry, the values advocated (Burton and Macdougall, "Analysis of the Problem of Protection of the Aviator Against Cold and the Test- ing of the Insulating Power of Flying Clothing," August 11, 1941, A.C.A.M.R.⢠Report No. C 2035 (N.R.C. Canada) are based upon the work of the Pierce Laboratory on the heat losses of the naked and the clothed human body with various air move- ments (Winslow, Gagge and Herrington, Am. J. Physiol. 127, 505, 1939)⢠The data are described by an equation, which converted to Clo units, is.; IA = 0.61 A. 19 V C1° . (2) where V is the velocity of air movement in cms./sec. In this equation the term 0.19 V represents the loss by convection, the other term 0.61, which is not de- pendent on air movement, the loss by radiation. The data were obtained at ordinary room temperatures (25°C.). The equation can be modified for low temperatures in accordance with the. foregoing discussion to take account of the changed radiation factor and the changed density of the air. It becomes: 1 0.61 x (^g)3 + 0-19 V a where T is the ambient temperature. For example, for -40°F., T = 233°K. and the equation becomes: IA = 0.29 + 0.21 V ' ^ ' From equations (2) and (4) a table of values of the insulation of the air may be constructed, for ordinary room temperatures and for low temperatures (-40°F.) respectively. From the relation of the two terms in the denominator in equation (4) the percentages of the total heat loss by radiation and by convection (for clothing of high emissivity) may be estimated. These are also given in Table 1. It will be seen that in relatively still air, the insulation increases at low temperatures, while in high air,movements it decreases at low tempera- tures. However, over the working range of laboratory and field experiments, from 30 cm./sec. up, the values differ by less than 0.1 Clo unit. The similarity of the two curves is shown in Fig. 1. Standard Values Recommended Since the accuracy of Clo determinations cannot be expected to be better than to 0.1 Clo units or more, due to the variables introduced by tightness of fit of clothing (an air space of .£â¢=- gives an extra 0.1 Clo), it is recommended that the effect of low ambient temperatures on the insulation of the air may be
TRANSFER OF HEAT TO THE AMBIENT AIR, AND THE THERMAL INSULATION Table 1 Air Movement Room Temp. 25°C. -40°C. cm. /sec, , ft./min. Insula- tion Insula- tion Rad'n. Con- Rad'n. Con- vection Clo vection Clo 9 18 52 48 0.85 32 68 1.08 .25 49 39 61 0.64 22 78 0.75 36 71 35 65 0.57 19 81 0.64 49 96 31 69 0.52 16 84 0.57 81 159 26 74 0.43 13 87 0.46 121 238 23 77 0.37 11 89 0.39 225 443 18 82 0.29 8 92 0.29 400 790 14 86 0.23 6 94 0.22 625 1,230 11 89 0.19 5 95 0.18 1,024 2,010 9 91 0.15 4 96 0.14 2,500 4,920 6 94 0.10 3 97 0.09 neglected in practice. A mean curve may be drawn falling between the two curves of Pig. 1, and from it a table of standard values made. These will be accurate to within about 0.05 Clo over the working range. ⢠Table 2 Standard Values of JA to be Used at all Temperatures Insulation Clo Units 0.1 0.15 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.85 Air Movement Ft./Min. 4 ,500 2,000 1,050 425 210 120 75 50 35 30 Cm. /Sec. 2 ,280 1,015 534 216 107 61 28 25 18 15 Measurement of Air Movement It must be emphasized that the velocity of random air movement is required rather than "drift velocity" of wind. The ordinary type of anenometer with rotat- ing cups or vanes is not really a suitable instrument for the purpose. The best method is to use a Katathermometer, which measures directly the cooling power of the air movement, rather than the velocity of this which is deduced from it. A method making use of an ordinary field thermometer, instead of the Katathermometer, has just been devised and will be reported separately. This has a sounder basis for application to the problem here discussed than has the Katathermometer, which, to our knowledge, was never calibrated for low tempera- tures.
CLOTHING TEST METHODS 1 O o o o
STANDARDIZATION OF SUBJECTS John H. Talbott Proper standardization of subjects is of great importance when one is testing tolerance time, dexterity, functional utility, morale and similar fac- tors. When determination of insulation units (clo) is planned, standardization is less important. The number of subjects should be adequate for proper statistical analy- sis. Their previous exposure history should be stated. If only one or two sub- jects are used daily for an extensive series of experiments, this should be noted. The other extreme is the employment of twenty or thirty enlisted men for a prolonged experiment (two or three months) following which they are returned to their organization for a new or previous assignment and are not employed as experimental subjects again. A satisfactory system that is in vogue at the Climatic Research Laboratory is a group of six or eight subjects participating daily for eight or ten weeks in the testing and then relieved by assignment to a non-testing job for a month, following which they are reassigned for cold-room testing. Selection An attempt should be made to select experimental subjects who are not old,er than thirty years of age. In the medical history there should be no record of having been intolerant to either heat or cold, nor having had any previous un- toward results following exposure to climatic extremes. Mild frostbite is not a contraindication to a person being a suitable candidate for cold-room studies. On the other hand, cold allergy is a contraindication. Extensive frostbite may render a previously resistant subject intolerant to the cold and any person with such a history should be selected only after careful consideration. In the high temperature range, sunstroke (heat hyperpyrexia) is a contraindication. Heat prostration and heat cramps are temporary exaggerated physiological responses to the heat and should not exclude a subject from selection. The region of the United States in which subjects are born and raised should not bias a physiological reaction, to either climatic extreme. Cold-re- sistant subjects are as likely to have been domiciled in the southern part of this country as elsewhere. The reverse is probably also valid. Training Trained subjects are to be preferred in all laboratory testing. The pe- riod of training may vary considerably depending upon the intelligence, coopera- tion and background of the subjects as well as upon the nature of the experiment. An identical or similar experiment should be experienced by each subject before acceptable data are collected. Acclimation Only acclimated subjects should be used for testing. Acclimation to the cold should involve at least ten days exposure, a minimum of four hours each day, before'.reportable results are to be collected. This length of time is usually needed for the first stage of acclimation to become effective. Beyond this time repeated exposure may increase acclimation but these changes are
